Merrill Genuine R-121R Fine Thread Complete Head Free ShippingGenuine Merrill R 121R complete fine threaded head for Merrill R6000 Yard Hydrant. The R 121R looks identical to the model R 121, however the threads that connect the head to the pipe on the R 121R are fine thread. This head comes complete, and is made of cast iron. When replacing the head it is a good idea to replace the plunger since the head is already off. Merrill R121R.